Qubit network non-volatile identification

ABSTRACT

A technique relates to a superconducting chip. Resonant units have resonant frequencies, and the resonant units are configured as superconducting resonators. Josephson junctions are in the resonant units, and one or more of the Josephson junctions have a shorted tunnel barrier.

DOMESTIC PRIORITY

This application is a continuation of U.S. application Ser. No.15/598,928, titled “QUBIT NETWORK NON-VOLATILE IDENTIFICATION”, filedMay 18, 2017, the contents of which are incorporated by reference hereinin its entirety.

BACKGROUND

The present invention generally relates to superconducting devices. Morespecifically, the present invention relates to qubit networknon-volatile identification.

Non-volatile memory, nonvolatile memory, NVM, or non-volatile storage isa type of computer memory that can retrieve stored information evenafter having been power cycled (i.e., repeatedly turned off and backon). Examples of non-volatile memory include read-only memory, flashmemory, ferroelectric random access memory, most types of magneticcomputer storage devices (e.g., hard disk drives, floppy disks, andmagnetic tape), optical discs, and early computer storage methods suchas paper tape and punched cards. Non-volatile memory is typically usedfor the task of secondary storage or long-term persistent storage. Themost widely used form of primary storage today is a volatile form ofrandom access memory (RAM), which means that when the computer is shutdown, anything contained in RAM is lost. Non-volatile data storage canbe categorized in electrically addressed systems (read-only memory) andmechanically addressed systems (hard disks, optical disc, magnetic tape,holographic memory, and such).

In computing, eFUSE is a technology invented by IBM® which allows forthe dynamic real-time reprogramming of computer chips. Computer logic isgenerally “etched” or “hard-coded” onto a chip and cannot be changedafter the chip has finished being manufactured. By utilizing a set ofeFUSEs, a chip manufacturer can allow for the circuits on a chip tochange while it is in operation. Additionally, eFUSEs can be utilized toidentify the chip or to store information about faulty bits in othermemory, for the purpose of replacing them with redundant ones, to cite afew applications.

New ways of creating identifications using non-volatile memory/devicesare needed, in particular, memory elements compatible withnon-traditional computing platforms. As the number of systems carryingsuperconducting chips operated at cryogenic temperatures increases, sodoes the need for non-volatile memory for such chips. Distributingworkloads across a network of such chips can be accomplished byidentification, which in turn can be implemented with non-volatilememory. But there are scarce simple non-volatile memories that arecompatible with operation at cryogenic temperatures.

SUMMARY

Embodiments of the present invention are directed to a superconductingchip. A non-limiting example of a superconducting chip includes resonantunits having resonant frequencies, where the resonant units areconfigured as superconducting resonators, and Josephson junctions in theresonant units. One or more of the Josephson junctions have a shortedtunnel barrier.

Embodiments of the present invention are directed to a method of forminga superconducting chip. A non-limiting example of the method of formingthe superconducting chip includes providing resonant units havingresonant frequencies, Josephson junctions being in the resonant units,and causing one or more of the Josephson junctions to have a shortedtunnel barrier.

Embodiments of the present invention are directed to a method ofidentifying a superconducting chip. A non-limiting example of the methodof identifying the superconducting chip includes receiving firstresonant frequencies and second resonant frequencies. The first resonantfrequencies are in a predefined frequency band, and the second resonantfrequencies are outside of the predefined frequency band. The secondresonant frequencies each have an expected frequency location in adifferent predefined frequency band. The method includes correlating afirst representation to each of the first resonant frequencies in thepredefined frequency band, and correlating a second representation tothe expected frequency location for each of the second resonantfrequencies, where a combination of the first and second representationsidentify the superconducting chip.

Embodiments of the present invention are directed to a method of causingidentification of a superconducting chip. A non-limiting example of themethod of causing identification of the superconducting chip includescausing the superconducting chip to provide resonant frequencies fromresonant units. The resonant frequencies include first resonantfrequencies within a predefined frequency band and second resonantfrequencies outside of the predefined frequency band, where the secondresonant frequencies each have an expected frequency location in adifferent predefined frequency band. The method includes correlating afirst representation to each of the first resonant frequencies in thepredefined frequency band and a second representation to the expectedfrequency location for each of the second resonant frequencies, where acombination of the first and second representations is a presentidentification of the superconducting chip. Also, the method includesdetermining that a previously stored identification is a match to thepresent identification of the superconducting chip.

Embodiments of the present invention are directed to a superconductingchip. A non-limiting example of the superconducting chip includes one ormore non-shorted Josephson junctions in first resonant units. The firstresonant units have first resonant frequencies, where the one or morenon-shorted Josephson junctions cause the first resonant frequencies tobe present in a predefined frequency band. The predefined frequency bandis determined in advance. Also, the superconducting chip includes one ormore shorted Josephson junctions in second resonant units. The secondresonant units have second resonant frequencies, where the one or moreshorted Josephson junctions cause the second resonant frequencies to beabsent from the predefined frequency band. An identification of thesuperconducting chip includes representation of both a presence of thefirst resonant frequencies and an absence of the second resonantfrequencies in the predefined frequency band.

Additional technical features and benefits are realized through thetechniques of the present invention. Embodiments and aspects of theinvention are described in detail herein and are considered a part ofthe claimed subject matter. For a better understanding, refer to thedetailed description and to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The specifics of the exclusive rights described herein are particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other features and advantages ofthe embodiments of the invention are apparent from the followingdetailed description taken in conjunction with the accompanying drawingsin which:

FIG. 1 depicts a schematic of an identification system that providesidentification of superconducting qubit chips according to embodimentsof the present invention;

FIG. 2 depicts a schematic of the identification system that providesidentification of superconducting qubit chips according to embodimentsof the present invention;

FIG. 3 depicts a schematic of an identification system where resonantunits are addressed collectively according to embodiments of the presentinvention;

FIG. 4 depicts a schematic of the identification system where resonantunits are individually addressed according to embodiments of the presentinvention;

FIG. 5A depicts a schematic of a resonant unit configuration accordingto embodiments of the present invention;

FIG. 5B depicts a schematic of a resonant unit configuration accordingto embodiments of the present invention;

FIG. 5C depicts a schematic of a resonant unit configuration accordingto embodiments of the present invention;

FIG. 5D depicts a schematic of a resonant unit configuration accordingto embodiments of the present invention;

FIG. 5E depicts a schematic of a resonant unit configuration accordingto embodiments of the present invention;

FIG. 5F depicts a schematic of a resonant unit configuration accordingto embodiments of the present invention;

FIG. 5G depicts a schematic of a resonant unit configuration accordingto embodiments of the present invention;

FIG. 5H depicts a schematic of a resonant unit configuration accordingto embodiments of the present invention;

FIG. 6A depicts an example resonant unit array according to embodimentsof the present invention;

FIG. 6B depicts an example resonant unit array having been programmedaccording to embodiments of the present invention;

FIG. 6C depicts a frequency spectrum for non-programmed resonant unitarray according to embodiments of the present invention;

FIG. 6D depicts a frequency spectrum for a programmed resonant unitarray according to embodiments of the present invention;

FIG. 7A depicts an example resonant unit array according to embodimentsof the present invention;

FIG. 7B depicts an example resonant unit array having been programmedaccording to embodiments of the present invention;

FIG. 8 depicts example authentication of a superconducting chipaccording to embodiments of the present invention;

FIG. 9 depicts a flow chart of a method of forming a superconductingchip according to embodiments of the present invention.

FIG. 10 depicts a flow chart of a method of identifying asuperconducting chip according to embodiments of the present invention;

FIG. 11 depicts a flow chart of a method of causing identification of asuperconducting chip according to embodiments of the present invention;

FIG. 12A depicts a partial view of a resonant unit having a non-shortedJosephson junction according to embodiments of the present invention;

FIG. 12B depicts a partial view of the resonant unit having a shortedJosephson junction according to embodiments of the present invention;

FIG. 13A depicts a partial view of a resonant unit having a non-shortedJosephson junction according to embodiments of the present invention;and

FIG. 13B depicts a partial view of the resonant unit having a shortedJosephson junction according to embodiments of the present invention.

The diagrams depicted herein are illustrative. There can be manyvariations to the diagram or the operations described therein withoutdeparting from the spirit of the invention. For instance, the actionscan be performed in a differing order or actions can be added, deletedor modified. Also, the term “coupled” and variations thereof describeshaving a communications path between two elements and does not imply adirect connection between the elements with no interveningelements/connections between them. All of these variations areconsidered a part of the specification.

In the accompanying figures and following detailed description of thedisclosed embodiments, the various elements illustrated in the figuresare provided with two or three digit reference numbers. With minorexceptions, the leftmost digit(s) of each reference number correspond tothe figure in which its element is first illustrated.

DETAILED DESCRIPTION

For the sake of brevity, conventional techniques related tosemiconductor device and integrated circuit (IC) fabrication may or maynot be described in detail herein. Moreover, the various tasks andprocess steps described herein can be incorporated into a morecomprehensive procedure or process having additional steps orfunctionality not described in detail herein. In particular, varioussteps in the manufacture of semiconductor devices andsemiconductor-based ICs are well known and so, in the interest ofbrevity, many conventional steps will only be mentioned briefly hereinor will be omitted entirely without providing the well-known processdetails.

Turning now to an overview of technologies that are more specificallyrelevant to aspects of the invention, numerous chips are utilized inserver farms. A server farm or server cluster is a collection ofcomputer servers, usually maintained by an organization to supply serverfunctionality far beyond the capability of a single machine. Serverfarms often consist of thousands of computers. As superconductingquantum computing hardware production scales up, a need emerges formeans to identify chips in a superconducting quantum computing network(e.g., numerous superconducting quantum computers like a server farm inwhich each quantum computer has at least one superconducting chip) whilein operation. For a small number of packaged chips, an inventory can bemade to track computing resources. For a network, it is useful anddesirable to be able to identify the superconducting chip itself.Ideally, this needs to be done in the same environment as the functionalhardware and using the same type of measurement tools. In complementarymetal-oxide-semiconductor (CMOS), this is usually accomplished by theaddition of NVRAM, such as eFUSE. In eFUSE, bits are programmed during aregistration phase prior to deployment in the field, and NVRAM does notlose information after power is turned off (as opposed to static randomaccess memory (SRAM) or dynamic random access memory (DRAM)).

Turning now to an overview of the aspects of the invention, one or moreembodiments of the invention address the above-described shortcomings ofthe prior art by providing qubit network non-volatile identification. Inaccordance with embodiments of the invention, a quantum analog forsuperconducting quantum chips is provided which requires programmablebits that can be read out using microwave circuitry. This type of NVRAMminimizes the use of precious real-estate (i.e., space or area) on achip. More specifically, the above-described aspects of the inventionaddress the shortcomings of the prior art by providing an authenticationsystem that can be readily integrated with superconducting qubit chipfabrication and can be read out with the same equipment and techniquesfor typical superconducting qubit chips. Superconducting tunneljunctions (also referred to as Josephson junctions) used insuperconducting quantum chips exhibit a Josephson inductance when in thesuperconducting state, which at low-to-moderate powers is functionallyequivalent to a conventional inductor within the circuit. Embodiments ofthe invention are configured to eliminate the Josephson inductance (byshort circuiting) of Josephson junctions in desired resonant units toconfigure an identification for the superconducting chip.

Superconducting tunnel junctions (Josephson junctions) used insuperconducting chips are susceptible to shorting through the tunnelbarrier. The tunnel barrier is often a dielectric material. In thesuperconducting state, a Josephson junction exhibits a Josephsoninductance as noted above. Shorting the Josephson junction can be causedby electrostatic discharge (ESD) or a high enough current for Josephsonjunctions such that breakdown of the thin barrier material occurs. Thisbreakdown causes a permanent short (i.e., short circuit) between the twosuperconducting electrodes (e.g., superconducting metal) that sandwichthe tunnel junction. Accordingly, this provides a path under which thesetunnel junctions can be programmed as a short, therefore creating asubstantially different resistance state (and inductance) from theinitially-fabricated junction according to embodiments of the invention.As such, resonant units having shorted Josephson junctions and regularJosephson junctions (i.e., non-shorted Josephson junctions) can be readout to thereby provide a non-volatile identification of thesuperconducting quantum chip. The quantum analog for superconductingquantum chips provides an identification that can be read out usingmicrowave circuitry just as a typical superconducting qubit. Theidentification allows for each superconducting quantum chip in a(superconducting) server farm/network to be uniquely identified in situ(i.e., under superconducting temperatures such as in a dilutionrefrigerator).

Turning now to a more detailed description of aspects of the presentinvention, FIG. 1 depicts a schematic of an identification system 100configured to provide programmed non-volatile identification ofsuperconducting chips where readout is collectively performed accordingto embodiments of the present invention. FIG. 2 depicts a schematic ofthe identification system 100 configured to provide programmednon-volatile identification of superconducting chips where readout isindividually addressable according to embodiments of the presentinvention.

The identification system 100 includes a superconducting chip 102. Thesuperconducting chip 102 includes a resonant unit array 104 along withother types of circuitry utilized for quantum computing. For example,other circuitry can include superconducting qubit circuitry 130 utilizedfor superconducting quantum computing as understood by one skilled inthe art. There are various ways to perform quantum computing viasuperconducting qubit circuitry 130, and typical superconducting qubitcircuitry 130 will include the superconducting qubits, readoutresonators, coupling resonators, coupling capacitors, coupling inductorsand other superconducting circuit elements used for quantum computation.The combination of all these necessary elements is understood by oneskilled in the art, and the details are not discussed herein. Thesuperconducting chip 102 operates at superconducting temperatures. Thesuperconducting chip 102 can be cooled by a cryogenic device (not shown)such as a dilution refrigerator.

The resonant unit array 104 includes resonant units 150_1 through 150_N.Each resonant unit 150_1 through 150_N contains a Josephson junction,along with a capacitor, and an inductor, either in lumped or distributedform. Various configurations of an individual resonant unit 150 areillustrated in FIGS. 5A, 5B, 5C, 5D, 5E, 5F, 5G, and 5H. Each resonantunit 150_1 through 150_N is a resonator with a unique resonant (orresonance) frequency that can be read out in the same manner that atypical superconducting qubit circuitry 130 is read out. For example,the resonant units 150_1 through 150_N have individual resonantfrequencies f1 through fN. Typical superconducting qubit circuitry 130are made and utilized for quantum computations and/or quantum operations(such as entanglement, etc.).

The resonant frequency of each resonant unit 150_1 through 150_N can beread out using measurement equipment 106, which is the same equipmentused to read out typical superconducting qubit circuitry 130. Themeasurement equipment 106 is operatively connected to resonant units150_1 through 150_N of the resonant unit array 104 on chip 102 viatransmission line 120. Unlike FIG. 1, FIG. 2 depicts an example in whicheach of the resonant units 150_1 through 150_N is individuallyaddressable via transmission lines 120_1 through 120_N. The transmissionlines 120_1 through 120_N are feedlines for transmitting and receivingsignals (e.g., microwave signal). The transmission lines 120 can becoaxial lines or waveguides. The transmission lines are one type oftransmission medium. Another example transmission medium can be athree-dimensional microwave cavity, which can be utilized for readout ofthe resonant unit array 104. The transmission medium and resonant units150_1 through 150_N share the same chip 102 with typical superconductingqubit circuitry 130 and other superconducting circuit elements used forquantum computation. Therefore, any implementation must prevent theleakage of quantum information from the superconducting qubit circuitry130 into the resonant unit array 104. In one implementation, theresonant unit array 104 is strictly separated from the superconductingqubit circuitry 130 by distance on the chip 102 and by groundedshielding incorporated into the chip 102, so as to prevent anycapacitive or inductive coupling between the two types of circuitry 104and 130. In this case, the resonant units 150_1 through 150_N can haveresonant frequencies f1 through fN of any value. In anotherimplementation, capacitive or inductive coupling can exist between thetwo types of circuitry 104 and 130. In this case, the resonant units150_1 through 150_N must be made so that their resonant frequencies f1through fN do not overlap the frequencies of the superconducting qubitcircuitry 130 utilized for quantum computing. Another implementation inwhich resonant frequencies f1 through fN might overlap the frequenciesof the superconducting qubit circuitry 130 is to add a switch thatconnects to each of the resonant units 150_1 through 150_N to groundwhen closed. The switch is closed during the superconducting operationof the superconducting qubit circuitry 130. When open, the resonantunits 150_1 through 150_N are not connected to ground and thesuperconducting qubit circuitry 130 is not performing quantum computing.

The measurement equipment 106 can include a probe signal 132. The probesignal 132 can be produced by a signal generator (e.g., integrated inthe measurement equipment 106) configured to generate radio frequencysignals (which can be pulses) at desired radio frequencies such as, forexample, microwave signals. In order to read the identification of theresonant units 150_1 through 150_N, the probe signal 132 is configuredto generate microwave signals to sweep the frequencies across thedesired/predefined frequencies and transmit to the resonant units 150_1through 150_N of the resonant unit array 104. FIG. 1 depicts that theprobe signal 132 transmits the radio frequency signal (being a sweep ofthe desired radio frequencies) to the resonant units 150_1 through 150_Ncollectively (via transmission line 120). In other words, themeasurement equipment 106 is configured to transmit the radio frequencysignal (covering all of the predefined frequencies) to each of theresonant units 150_1 through 150_N at once using the same transmissionmedium (e.g., transmission line 120 or a microwave cavity). FIG. 2depicts that the probe signal 132 transmits the microwave signal (beinga sweep of the desired frequencies) to the resonant units 150_1 through150_N individually (via transmission lines 120_1 through 120_N). In FIG.2, the resonant units 150_1 through 150_N can be isolated and eachaccessed via an individual transmission line, such that the radiofrequency signal is switched between these transmission lines 120_1through 120_N. In one implementation, the radio frequency signal (beinga sweep of the desired frequencies) can be simultaneously or nearlysimultaneously transmitted on each individual transmission line 120_1through 120_N to the respective resonant units 150_1 through 150_N.Because the measurement equipment 106 via the probe signal 132 isconfigured to generate radio frequency signals with predefinedfrequencies to individually address the resonant units 150_1 through150_N via individual transmission lines 120_1 through 120_N as depictedand because resonant units 150_1 through 150_N have individualtransmission lines 120_1 through 120_N, the resonant frequency can bethe same for each resonant units 150_1 through 150_N as long as theresonant units 150_1 through 150_N are individually read out.

In both cases (FIG. 1 and FIG. 2), the radio frequency signal isdirected at resonant units 150_1 through 150_N individually and/ordirected at the entire array of resonant units 150_1 through 150_N(collectively). In advance, the measurement equipment 106 knows theexpected/predefined resonant frequencies for resonant units 150_1through 150_N such that the frequency range (or frequency band) of theradio frequency signal (transmitted to the resonant unit array 104) isintended to match/coincide with the expected resonant frequencies forresonant units 150_1 through 150_N. The measurement equipment 106 isconfigured to operate automatically (via processors executing computerinstructions) and/or with assistance from an operator.

Although transmission lines 120_1 through 120_N are shown as thechannels for directing the radio frequency signal (and likewisereceiving responses back from the resonant units 150_1 through 150_N),the channel can be a three-dimensional microwave cavity as understood byone skilled in the art. Also, each transmission line 120 can berepresentative of two transmission lines when readout is intransmission.

While considering resonant unit 150_1 for explanation purposes, itshould be appreciated that the following discussion applies by analogyfor each of the resonant units 150_2 through 150_N. If the frequency ofradio frequency signal (i.e., the challenge) equals the resonantfrequency of resonant unit 150_1, this will be evident in the phase oramplitude of the radio frequency energy reflected or transmitted fromresonant unit 150_1 through the transmission line 120 (or transmissionline 120_1). For example, assuming that the resonant unit 150_1 has aresonant frequency f1, the response/return radio frequency energy/signalwill have a peak in amplitude at frequency f1 and a 180 degree phaseshift centered at frequency f1. To read the full identification(fingerprint) of the resonant unit array 104, the probe signal 132 mustsweep the frequency of the radio frequency signal over the range ofresonant frequencies present in all the resonant units 150_1 through150_N (for example, from 3 gigahertz (GHz) to 10 GHz) and the probesignal must direct the swept radio frequency signal at all of theresonant units 150_1 through 150_N. Accordingly, all the resonant units150_1 through 150_N will return radio frequency energy/signals having apeak in amplitude at their respective resonant frequencies f1 through fNand a 180 degree phase shift centered at respective resonant frequenciesf1 through fN. Although peaks in the frequency spectrum are utilized forexplanation purposes, it should be noted that the measurement is notlimited to measuring peaks. In some implementations, each of the peakscan be a dip depending on the measurement and other system parameters,such that identification is based on measuring dips.

As the response to the previously transmitted radio frequency signal tothe resonant units 150_1 through 150_N, the measurement equipment 106can receive a sequence of the resonant frequencies f1 through fN wherethe frequency response is utilized as a binary representation of zeros(0) and ones (1) as the identification of the chip 102 as discussedfurther herein. The sequence of the resonant frequencies f1 through fNcan be stored in the memory 112 and/or be stored separately. Afterreceiving the response (returned signals) from resonant units 150_1through 150_N of the resonant unit array 104, the measurement equipment106 is configured to perform a spectral analysis to determine/identifythe spectrum of frequencies (peaks) in frequency space for the responsereceived from the resonant units 150_1 through 150_N. In oneimplementation, the measurement equipment 106 is configured to identifythe resonant frequencies f1 through fN for each of the resonant units150_1 through 150_N as the peaks at frequencies f1 through fN within apredefined frequency band. The predefined frequency band is a range offrequencies known (expected) in advance by the measurement equipment106.

In some embodiments of the present invention, the chip 102 iscryogenically cooled and its resonance frequencies are measured afterfabrication to test that the resonant units 150_1 through 150_N arefunctioning. This spectrum is stored as a reference. The chip 102 isthen programmed (i.e., desired bits are shorted). The chip 102 is thenused in quantum computing operations (i.e., during cooling). When thechip's identification is required, the spectrum is measured and comparedto the reference spectrum. Large deviations in any resonance linesindicate a flipped bit.

In other embodiments, the chip 102 is fabricated, and the bits areprogrammed (before cooling). The chip is then cryogenically cooled andmeasured to produce a reference spectrum, which is enrolled (i.e.,stored) in the chip ID database 110. The chip 102 is then used inquantum computing operations (i.e., during cooling). When the chip'sidentification is required during authentication, the spectrum ismeasured. A match against the reference spectrum identifies the chip102.

The sequence of the resonant frequencies f1 through fN can betransmitted from the measurement equipment 106 to a computer system 108via a communication medium 122. The communication medium 122 can be awired (Ethernet cable, USB cable, optical fiber cable, coaxial cable,and twisted pair cable, etc.) or a wireless connection. The computersystem 108 has one or more processors. Similarly, the measurementequipment 106 can have one or more processors. The computer system 108is configured to compare the binary representation (derived from thesequence of the resonant frequencies f1 through fN including the absenceof any resonant frequencies in the predefined frequency band) receivedfrom the measurement equipment 106 with various chip identificationnumbers previously stored for similar chips 102 in a chip identificationdatabase 110. The computer system 108 is configured to determine whetherthe received binary representation (derived from the sequence of theresonant frequencies f1 through fN within the predefined frequency bandand absent from the predefined frequency band) matches a previouslystored binary representation (from a sequence of the resonantfrequencies). The chip 102 can be representative of numerous chips. Thechip identification database 110 can include the chip identificationnumbers for numerous chips 102. Each chip 102 could be utilized in anetwork of superconducting quantum computers in order to perform quantumcomputing as understood by one skilled in the art. The respective chipidentification numbers for all chips 102 are readout and stored inadvance as a binary representation in the chip identification database110. Each chip 102 could have been read out and stored by themanufacturer of the chips 102. Also, each chip 102 could have been readout and stored by the operator (end user) of the chips 102 who hasdeployed the chips in the network of superconducting quantum computers.

In some embodiments of the present invention, the computer system 108can be integrated with the measurement device 106 as illustrated inFIGS. 3 and 4. FIG. 3 depicts a schematic of an identification system100 where the resonant units 150_1 through 150_N of the resonant unitarray 104 are addressed collectively through the transmission line 120according to embodiments of the present invention. In other words, theradio frequency signal (challenge) can be sent from the measurementequipment 106 over the same transmission line(s) to all of the resonantunits 150_1 through 150_N and the responses are received back over thesame transmission line(s). FIG. 4 depicts a schematic of theidentification system 100 where the resonant units 150_1 through 150_Nof the resonant unit array 104 are individually addressed through thetransmission lines 120_1 through 120_N according to embodiments of thepresent invention. In FIGS. 3 and 4, the measurement equipment 106 doesnot have to transmit the converted binary representation (or receivedsequence of the resonant frequencies f1 through fN) to the computersystem 108. Instead, the computer system 108 having the functionality ofthe measurement equipment (or vice versa) can immediately compare thereceived binary representation (of the sequence of the resonantfrequencies f1 through fN) to the chip identification numbers stored inthe chip identification database 110 and then identify the chip 102 aschip ID1 (or chip XYZ).

Now turning to more detail regarding the resonant units 150_1 through150_N, each resonant unit has at least one Josephson junction. Theresonant units 150_1 through 150_N are similar to typicalsuperconducting qubits but have less stringent requirements. A typicalsuperconducting qubit has to be manufactured such that it has longrelaxation and coherence times T1 and T2 and such that the qubit canretain its quantum state information. In typical superconducting qubits,the qubit state can be high |1>, low |0>, or a mathematicalsuperposition of both high and low. Additionally, typicalsuperconducting qubits utilized for quantum computing (such as quantumcomputing superconducting qubit circuitry 130) cannot be read outdirectly. Instead, a readout resonator is required to be read out suchthat the quantum information (i.e., state) of the typicalsuperconducting qubits can be inferred based at least in part on themicrowave signal received back from reading out the readout resonator.Typical superconducting qubits need coupling capacitors to separate eachtypical superconducting qubit from its readout resonator. Also, typicalsuperconducting qubits need coupling capacitors to separate the typicalsuperconducting qubits from one another. However, resonant units 150_1through 150_N do not need to maintain state information and thereforeare not limited by requirements to maintain long times T1 and T2. Also,the resonant units 150_1 through 150_N can be read out directly bycausing the resonant units 150_1 through 150_N to each resonate at theirrespective resonant frequencies f1 through fN. Additionally, resonantunits 150_1 through 150_N do not need to be separated from one anotheror separated from readout resonators (by coupling capacitors).Consequently, resonant units 150_1 through 150_N can be packed tightlytogether without an issue of losing state information because stateinformation is not needed and without an issue of interference amongeach other.

Similar to typical qubits, the resonant units 150_1 through 150_Nutilize Josephson junctions. A Josephson junction is formed by twosuperconductors coupled by, for example, a thin insulating barrier. AJosephson junction can be fabricated by means of an insulating tunnelbarrier, such as Al₂O₃, between superconducting electrodes. For suchJosephson junctions, the maximum supercurrent that can flow through thebarrier is the critical current I_(c).

FIGS. 5A, 5B, 5C, 5D, 5E, 5F, and 5G depict various resonant unitschematic configurations according to embodiments of the presentinvention. The resonant units 150_1 through 150_N can be implemented asany of the example in FIGS. 5A-5G. In FIGS. 5A-5G, each example resonantunit 150 includes a Josephson junction (JJ), an inductor L, and acapacitor C. The inductor L and capacitor C can include circuitcomponents designed to exhibit a particular amount of inductance andcapacitance, respectively, or can otherwise have the amount ofinductance and capacitance present within the metal patterns and wiringof the circuit, for instance for the case of on-chip transmission lines,as understood by those skilled in the art. Along with the Josephsonjunction, a resonant unit 150 can be formed by combining capacitors andother inductors on the chip 102. The line-width of the resonance isdetermined by its coupling to the feed-line or other read-out circuitry,and must be made small enough to clearly distinguish each resonance inthe frequency domain. As discussed herein, any Josephson junction can beconverted into a short by applying enough current to breakdown thebarrier material, therefore working similarly to a one-time programmableelement.

The Josephson junction acts as an inductor and therefore contributes tothe total inductance. Frequency addressability (i.e., the differentfrequencies for resonant units 150_1 through 150_N) can be enforced bychanging the amount of inductance and/or changing the amount ofcapacitance from one resonant unit 150 to another. For example, more orless series inductance and/or parallel inductance can be adjusted (i.e.,increased or decreased) from one resonant unit 150 to the next.Additionally, more or less series and/or parallel capacitance can beadjusted from one resonant unit 150 to the next. Larger or smallerJosephson inductance can be adjusted (i.e., increased or decreased) ineach resonant unit 150. Even if the inductance of the inductor L and thecapacitance of the capacitor C remain the same and the configuration(such as any configuration in FIGS. 5A-5G) is the same for each resonantunit 150_1 through 150_N, one or more of the resonant units haveJosephson junctions that are shorted in order to cause the resonantunits with the shorted Josephson junctions to have a resonant frequencyoutside the predefined frequency band, thereby providing the binaryidentification number based at least in part on a presence or absence ofresonant frequencies within the predefined frequency band.

Taking the configuration of FIG. 5A as an example, FIGS. 6A, 6B, 7A and7B illustrate example resonant unit arrays 104 according to embodimentsof the present invention. FIG. 6A depicts a schematic of a resonant unitarray 104 in which the resonant units 150_1 through 150_N arecollectively addressed and the Josephson junctions have not beenprogrammed. FIG. 6B depicts a schematic of a resonant unit array 104 inwhich the resonant unit 150_1 through 150_N are collectively addressedand Josephson junctions are programmed by inducing a short in a tunneljunction. In FIGS. 6A and 6B, the radio frequency signal is input to onetransmission line 120 of the resonant unit array 104. The resonant unitarray 104 can have only one transmission line 120 if operating inreflection. The resonant unit array 104 can have a second transmissionline 120 if operating in transmission, which is shown with dashed lines.The dashed lines transmits the response back to the measurementequipment 106.

FIG. 7A depicts a schematic of a resonant unit array 104 in which theresonant units 150_1 through 150_N are individually addressed and theJosephson junctions are not programmed. FIG. 7B depicts a schematic of aresonant unit array 104 in which the resonant unit 150_1 through 150_Nare individually addressed and Josephson junctions are programmed byinducing a short in a tunnel junction. In FIGS. 7A and 7B, the radiofrequency signal is input to each of transmission lines 120_1 through120_N of the resonant unit array 104. The resonant unit array 104 canhave only one set of transmission lines 120_1 through 120_N if operatingin reflection. The resonant unit array 104 can have a second set oftransmission lines 120_1 through 102_N if operating in transmission,which is shown with dashed lines. The dashed lines transmits theresponse back to the measurement equipment 106.

FIGS. 6A, 6B, 7A, and 7B show that the resonant units 150_1 through150_N include Josephson junctions 602_1 through 602_N, capacitors (C)604_1 through 604_N, and inductors (L) 606_1 through 606_N. In FIGS. 6A,6B, 7A, and 7B, the choice of resonant unit configuration is just anexample, but each contains at least one Josephson junction. The choiceof resonant unit 150 (and number of resonant units 150) is just anexample. In FIGS. 6A and 6B, transmission line 120 can be capacitivelycoupled to resonant units 150_1 through 150_N by coupling capacitors(CC) 608_1 through 608_N. In some implementations, coupling inductorscan be used in place of coupling capacitors. In other implementations,the coupling capacitors (CC) 608_1 through 608_N can be omitted. InFIGS. 6A and 7A, the capacitances of the capacitors 604_1 through 604_Nare the same in this example, such that C1=C2=C3 . . . =CN, and theinductances of the inductors 606_1 through 606_N are not the same, suchthat L1>L2>L3 . . . >LN, thereby resulting in resonant frequencies f1,f2, f3, . . . fN within the predefined frequency band (as shown in FIG.6C), where f1<f2<d3 . . . , fN. FIG. 6C depicts a graph 650 of thefrequency spectrum when the resonant unit array 104 is read out. FIG. 6Crepresents the readout of the resonant unit array 104 in FIGS. 6A and 7Avia the measurement equipment 106 before any programming of the resonantunits 150 (or Josephson junctions in the resonant units 150). In otherwords, the measurement equipment has transmitted radio frequency signalssweeping the frequency range (or frequency band) that matches/coincideswith the resonant frequencies f1 through fN, in order for themeasurement equipment 106 to receive the frequency response back fromthe resonant units 150_1 through 150_N shown in the frequency spectrumin FIG. 6C. As can be seen in the graph 650, each of the resonantfrequencies f1 though fN has a magnitude that peaks, and the peaks canbe greater than a predefined threshold. Although peaks in the frequencyspectrum are utilized for explanation purposes, it should be noted thatthe measurement is not limited to measuring peaks. In otherimplementations, each of the peaks can be a dip depending on themeasurement and other system parameters.

Magnitude can be representative of current, voltage, etc. When notprogrammed (i.e., no shorted Josephson junctions), the resonant units150_1 through 150_N are designed to resonate or to have their resonantfrequencies f1 through fN within a predefined frequency band. In someembodiments of the present invention, the predefined frequency band canbe about 3-6 GHz. In other embodiments of the present invention, thepredefined frequency band can be about 3-10 GHz. In some embodiments ofthe present invention, the predefined frequency band can be about 3-5GHz. Each of the resonant units 150_1 through 150_N can be considered abit. Also, each individual resonant frequency of resonant frequencies f1through fN can be considered a bit. When measurement equipment 106recognizes any of the resonant frequencies f1 through fN as a peak(e.g., above a predefined threshold) within the predefined frequencyband, the measurement equipment 106 reads each peak of the resonantfrequencies f1 through fN (or bit) as a 1. Each of the resonantfrequencies f1 through fN (or bits) is a 1 in FIG. 6C.

In FIGS. 6B and 7B, the capacitances of the capacitors 604_1 through604_N are the same in this example, such that C1=C2=C3 . . . =CN, andthe inductances of the inductors 606_1 through 606_N are not the same,such that L1>L2>L3 . . . >LN, thereby resulting in resonant frequenciesf1, f3, . . . fN within the predefined frequency band, where f1<f3 . . .fN<<<f2. However, in FIGS. 6B and 7B, the resonant unit 150_2 has beenprogrammed by shorting the Josephson junction 602_2 thereby causing theresonant frequency f2 to be outside of the predefined frequency band (asshown in FIG. 6D). The shorted Josephson junction 602_2 is depicted as astraight wire in the resonant unit 150_2 instead of an “X”.

FIG. 6D depicts a graph 652 of the frequency spectrum when the resonantunit array 104 is read out. Unlike FIG. 6C, FIG. 6D represents thereadout of the resonant unit array 104 in FIGS. 6B and 7B via themeasurement equipment 106 after programming of the resonant units 150(or Josephson junctions in the resonant units 150), and in this case,the resonant unit 150_2 is programmed (shorted Josephson junction602_2). In other words, the measurement equipment has transmitted radiofrequency signals sweeping the frequency range (or frequency band) thatmatches/coincides with the resonant frequencies f1 through fN, in orderfor the measurement equipment 106 to receive the frequency response backfrom the resonant units 150_1 through 150_N shown in the frequencyspectrum in FIG. 6D. As can be seen in the graph 652, each of theresonant frequencies f1, f3 though fN has a magnitude that peaks in thepredefined frequency band, and the peaks can be greater than apredefined threshold. Again, magnitude can be representative of current,voltage, etc. Because of the programming (i.e., shorted Josephsonjunction 602_2), the resonant frequency f2 of the resonant unit 150_2 isoutside the predefined frequency band and therefore does not peak withinthe predefined frequency band. The resonant frequency f2 is expected tobe located between the resonant frequencies f1 and f3 within thepredefined frequency band when not programmed, as identified by expectedfrequency location 660. The expected frequency location 660 is where thepeak for resonant frequency f2 would have been located if the Josephsonjunction 606_2 were not shorted. The measurement equipment 106 isconfigured to know in advance each of the expected frequency locationsfor each of the resonant frequencies f1 through fN, and to recognizewhen any peak is missing (just as for resonant frequency f2 which ismissing in this example).

When measurement equipment 106 recognizes the resonant frequencies f1,f3 through fN as a peak (e.g., above a predefined threshold) within thepredefined frequency band, the measurement equipment 106 correlates eachpeak of the resonant frequencies f1, f3 through fN (bits) as a 1 whilethe missing resonant frequency f2 (bit) corresponds to a 0. Themeasurement equipment 106 is configured to identify each of the resonantfrequencies f1 through fN (bits) as a 1 or 0 within the predefinedfrequency range, according to whether the corresponding resonant unit150_1 through 150_N has a shorted Josephson junction. In this example ofFIG. 6D (corresponding to readout of the resonant unit array 104depicted in FIGS. 6B and 7B), the binary sequence for thisidentification is 1011, where 1 corresponds to an intact junction and 0corresponds to a shorted junction (again this is an arbitrary conventionand can be vice versa).

The Josephson junctions can be nominally identical. The choice ofresonant unit 150_2 is being utilized as an example as the programmedresonant unit, but each resonant unit 150_1 through 150_N contains aJosephson junction, and one or more resonant units 150_1 though 150_Ncan be programmed as desired. The choice of element (and number ofelements) which is Josephson junction 602_2 to be changed foraddressability is just an example. In the example depicted in FIGS. 6Band 7B, the resonant unit 150_2 is programmed which means that theJosephson junction 602_2 is shorted. Accordingly, if inductance ofinductor L2 606_2 is much smaller than that of the Josephson junction602_2, the resonant frequency f2 becomes too large compared to theothers resonant frequencies f1, f3 through fN and is therefore outsideof the predefined frequency band checked for (i.e., considered) by themeasurement equipment 106. For explanation purposes, the response forresonant frequency f2 (which has been increased because of the shortcircuit (decreased Josephson inductance)) is shown as very high infrequency compared to the predefined frequency band. It is noted thatfrequency has an inverse relationship to inductance. More particularly,the frequency has an inverse square root relationship to inductance.

It should be appreciated that the resonant unit arrays 104 depicted inFIGS. 1-7 are for example. It should be noted that one or moreembodiments of the present invention can include a circuit (i.e., chip102) and/or system 100 having multiple resonant units 150_1 through150_N containing Josephson junctions 602_1 through 602_N, where theresonant units 150_1 through 150_N are coupled to a readout mechanism(such as measurement equipment 106) in order to read out a sequence ofanalog resonant frequencies signatures (i.e., bits), thereby providingan identification of the chip 102. The circuit can be a hanger-styletransmission line or a microwave cavity. In some implementations,individually addressed resonant units can be made with individualpatterned readouts (as depicted in FIGS. 2, 4, 7A, 7B). Although havingindividually addressed resonant units on chip 102 consumes more realestate, this allows though for the Josephson junctions (along withinductors L and capacitors C) to be made identical (i.e., with the sameresonant frequency) to one another in the resonant unit array 104,because Josephson junctions are addressed by their respective locationin their individual resonant units 150_1 through 150_N.

In other implementations, in the resonant unit array 104, the Josephsonjunctions (in respective resonant units 150_1 through 150_N) can be madeslightly different from one resonant unit 150 to the next. For example,the Josephson junctions 602_1 through 602_N (in respective resonantunits 150_1 through 150_N) can be made with different sizes, made withdifferent critical currents I_(c), made with different capacitive loads(i.e., different values for capacitors C1 604_1 through CN 604_N),and/or made with different inductive loads (i.e., different values forinductors L1 606_1 through LN 606_N) in order to identify each bit by apredictable analog frequency range. That is, one can have f1<f2<f3 . . .fN or vice versa for the resonant units 150_1 through 150_N. Each bit isa resonant frequency of the resonant units 150_1 through 150_N, suchthat the measured resonant frequency f1 is the bit for resonant unit150_1, measured resonant frequency f2 is the bit for resonant unit150_2, through measured resonant frequency fN is the bit for resonantunit 150_N. The chip 102 can represent numerous chips. A chip 102 canhave 128 bits thereby having 128 resonant units 150 such that there are128 resonant frequencies that are measured out. In one implementation,the resonant unit array 104 would utilize less space on the chip 102when the 128 resonant units 150 are addressed and measured collectivelybecause one transmission line 120 is needed for readout in reflection ortwo transmission lines 120 needed for readout in transmission. Having somany resonant units 150 (e.g., 128 bits, 64 bits, 32 bits, etc.) packedtightly is no problem for chip 102 because coherence is not a concernfor operation as noted above.

To illustrate authentication after readout of the chip 102, FIG. 8depicts an example of authentication of the chip 102 according toembodiments of the present invention. FIG. 8 only shows a simplifiedview of the system 100 so as not to obscure the figure. It should beappreciated that FIG. 8 includes all elements discussed in FIGS. 1-7. InFIG. 8, it is assumed that the identification of the chip 102 has beenread out and the measurement equipment 106 has identified the anyresonant frequencies f1 through fN that have peaks (at or above apredefined threshold) and have no peaks (below a predefined threshold)within the predefined frequency band. Once identification is read out,the measured identification can be checked (by the measurement equipment106/computer system 108) against the chip identification database 110.For a given number of resonant units 150, the chip 102 is read out withidentification 011010010101. The computer system 108 is configured tocompare this readout identification against each of the identificationnumbers in the chip identification database 110. Upon finding a match,the computer system 108 is configured to identify the chip 102 by itsidentification number. In the example of FIG. 8, the identification isrecognized as chip 1 (or chip XYZ) because the readouts are a match(i.e., the previously stored identification 011010010101 for ID1 matchesthe readout identification for chip 102).

The choice of Josephson junctions to be shorted can be selected by anoperator during the manufacturing/fabrication of the resonant units150_1 through 150_N. The manufacturer can fabricate numeroussuperconducting chips 102, such that each has a unique identificationnumber. Although the resonant unit 150_2 (having resonant frequency f2)has be utilized as an example in some scenarios, embodiments of thepresent invention can have any amount of Josephson junctions shortedwhile not having other Josephson junctions shorted.

As noted herein, there are many techniques for shorting Josephsonjunctions (i.e., shorting the tunnel barrier). FIG. 12A depicts apartial resonant unit 150 that only shows a non-shorted Josephsonjunction (e.g., any of the Josephson junction 602_1 through 602_N)according to embodiments of the present invention. FIG. 12A is anexample of using electrical probes with a high current to short theJosephson junction. In FIG. 12A, a power source 1202 is connected toboth sides of the Josephson junction via two probes 1204. The powersource 1202 can be a voltage source or current source for shorting theJosephson junction. One of the probes can be connected to a positivepolarity and the other probe can be connected to a negative polarity ofthe power source 1202. Electrical current is applied via probes 1204connected to power source 1202 such that the magnitude of the electricalcurrent breaks down the tunnel barrier of the Josephson junction,thereby causing the Josephson junction to be shorted. An example amountof current to short a Josephson junction can range from 100 μA to 100mA. After applying the electrical current, FIG. 12B depicts a partialresonant unit 150 that only shows the shorted Josephson junction (e.g.,any of the Josephson junctions 602_1 through 602_N can be shorted)according to embodiments of the present invention. In some embodimentsof the present invention, the Josephson junctions can be designed withprobing pads for the electrical probes 1204, and the probing pads can bemade custom for automated programming.

There are other techniques for shorting the Josephson junction. FIG. 13Adepicts a partial resonant unit 150 that only shows a non-shortedJosephson junction (e.g., any of the Josephson junction 602_1 through602_N) according to embodiments of the present invention. FIG. 13Aillustrates using the beam from, for example, an electron beam device1302 to short the Josephson junction. The electron beam device 1302 canbe a scanning electron microscope, electron beam gun, etc. In FIG. 13A,the electron beam device 1302 emits an electron beam 1304 on theJosephson junction. The electron beam 1304 is emitted with a magnitudethat creates electrical current in the Josephson junction suitable tobreak down the tunnel barrier of the Josephson junction, thereby causingthe Josephson junction to be shorted. After applying the electron beam,FIG. 13B depicts a partial resonant unit 150 that only shows the shortedJosephson junction (e.g., any of the Josephson junction 602_1 through602_N can be shorted) according to embodiments of the present invention.Any chip 102 can have its Josephson junctions shorted to generate a chipidentification number. There can be numerous superconducting chips 102in a server farm of quantum computers. One of the chips can have noshorted Josephson junctions, thereby resulting in a chip identificationnumber such as 1111, and this chip identification number isdistinguishable from other chips 102 having one or more shortedJosephson junction in the server farm.

Before discussing various flow charts of the superconducting chips 102below, and a generalized scenario for the superconducting chips 102 isnow discussed. A set/collection of identical superconducting qubit(circuit) chips 102 is fabricated. As noted herein, discussion of thesuperconducting chip 102 can represent numerous chips 102. A uniqueidentification code (i.e., chip identification) is programmed into eachone of the superconducting chips 102. Each of the superconducting chips102 can be installed into its own cryostat in the user's qubit-circuitfarm. A cryostat is a device, such as a dilution refrigerator, used tomaintain low cryogenic temperatures. While operating the numeroussuperconducting chips 102, the identification codes can be utilized todistinguish each among these otherwise-identical chips 102.

FIG. 9 depicts a flow chart 900 of a method of forming a superconductingchip 102 according to embodiments of the present invention. Variousoperations in FIG. 9 can be performed prior to installation in cryostat.At block 902, resonant units 150_1 through 150_N having resonantfrequencies are provided, and the resonant units are configured assuperconducting resonators. At block 904, Josephson junctions 602_1through 602_N are in the resonant units 150_1 through 150_N where one ormore of the Josephson junctions 602_1 through 602_N are caused to have ashorted tunnel barrier. An operator/manufacturer during fabrication ofthe resonant units 150_1 through 150_N can short any of the Josephsonjunctions 602_1 through 602_N. In one example, a Josephson junction canbe shorted by using electrical probes to apply electrical current tobreak down the tunnel barrier material in the Josephson junction.

The shorted tunnel barrier for the one or more of the Josephsonjunctions 602_1 through 602_N causes an increase in the resonantfrequencies for the resonant units having the shorted tunnel barrier.For example, as a result of the shorted tunnel barrier in Josephsonjunction 602_2, the resonant frequency f2 can been increased from, forexample, 4 GHz to 15 GHz. Resonant frequency f2 is only utilized as anexample. Some of the Josephson junctions have no shorted tunnel barrier.

The resonant frequencies (e.g., resonant frequencies f1, f3, f5, f7, f9,etc.) are designed to fall within a predefined frequency band for theresonant units having no shorted tunnel barrier in the Josephsonjunctions. The resonant frequencies (e.g., resonant frequencies f2, f4,f6, f8, f10, etc.) for the one or more of the resonant units having theshorted tunnel barriers are designed to fall outside of the predefinedfrequency band.

The one or more of the Josephson junctions having the shorted tunnelbarrier are predefined in advance in order to provide a predefined chipidentification. The Josephson junctions in the superconducting chips 102are not accidentally shorted and are not in arbitrary Josephsonjunctions. The Josephson junction is structured to have a firstconfiguration or a second configuration, wherein the first configurationis a typical Josephson junction defined as not being a short and thesecond configuration is the shorted Josephson junction. The firstconfiguration is a non-shorted Josephson junction, and the non-shortedJosephson junction has a tunnel barrier defined as not being shorted.The second configuration is a shorted Josephson junction, and theshorted Josephson junction is defined as having the shorted tunnelbarrier.

A combination of the first configuration and the second configurationdefines a binary representation, for example 101010101. A combination ofthe resonant frequencies, associated with first configuration and thesecond configuration, defines a binary representation. The resonantunits having the Josephson junctions without the shorted tunnel barrierare configured to be read out as a first binary number (e.g., a “1”).The resonant units having the one or more of the Josephson junctionswith the shorted tunnel barrier are configured to be read out as asecond binary number (e.g., a “0”).

FIG. 10 is a flow chart 1000 of a method of identifying a chip 102according to embodiments of the present invention. At block 1002, firstresonant frequencies (e.g., resonant frequencies f1, f3, f5, f7, f9,etc.) and second resonant frequencies (e.g., resonant frequencies f2,f4, f6, f8, f10, etc.) are received by the measurement equipment 106.The first resonant frequencies are in a predefined frequency band, andthe second resonant frequencies are outside of the predefined frequencyband, where the second resonant frequencies each have an expectedfrequency location (just like expected frequency location 660) in thepredefined frequency band. It should be noted that this is done whilethe chip 102 is installed in a cryostat for qubit operation, and thisaction can employ systems or equipment for sending/receiving microwavesignals that are same or identical to the systems/equipment used forreading qubits.

At block 1004, the measurement equipment 106 (and/or the computer system108) is configured to correlate a first representation (e.g., 1) to eachof the first resonant frequencies in the predefined frequency band. Atblock 1006, the measurement equipment 106 (and/or the computer system108) is configured to correlate a second representation (e.g., 0) to theexpected frequency location for each of the second resonant frequencies,wherein a combination of the first and second representations (ones (1)and zeros (0)) identify the chip 102.

The first representation is selected from the group consisting of anumber, a symbol, and/or a letter (e.g., X). The second representationis selected from the group consisting of another number, another symbol,and/or another letter (e.g., Y).

FIG. 11 is a flow chart 1100 of a method of causing identification of achip 102 according to embodiments of the present invention. At block1102, the measurement equipment 106 is configured to cause the chip 102to provide resonant frequencies f1 through fN from resonant units 150_1through 150_N, where the resonant frequencies include first resonantfrequencies (e.g., resonant frequencies f1, f3, f5, f7, f9, etc.) withina predefined frequency band and second resonant frequencies (e.g.,resonant frequencies f2, f4, f6, f8, f10, etc.) outside of thepredefined frequency band. The second resonant frequencies each have anexpected frequency location (just like the expected frequency locationfor f2) in a different predefined frequency band.

At block 1104, the measurement equipment 106 is configured to correlatea first representation (e.g., 1) to each of the first resonantfrequencies in the predefined frequency band and a second representation(e.g., 0) to the expected frequency location (just like for theexpected/missing frequency location for f2) for each of the secondresonant frequencies, where a combination of the first and secondrepresentations is a present identification of the chip 102.

At block 1106, the measurement equipment 106 (and/or the computer system108) is configured to determine that a previously stored identification(in the chip identification database 110) is a match to the presentidentification of the chip 102. It should be noted that the chipidentification can work (i.e., find a match) even if the second resonantfrequencies (outside of the predefined frequency band) are not clearlymeasured by the measurement equipment 106, i.e. if only theseresonances' absence from the whole set is observable.

In response to the match, the measurement equipment 106 (and/or thecomputer system 108) is configured identify the chip 102 having thepresent identification from different chips (in the database 110) havingdifferent identifications.

There are many scenarios of how the superconducting chips 102 can beutilized, and identification of a particular chip 102 among other chips102 can include various processes. First, after chip manufacturing, andoften performed at the manufacturer's site, the chip identification iscreated (by an operator who programs the chip identification into thechip 102) and stores it in the chip identification database 110. Thisprocess is typically known as enrollment. After the chip 102 is deployedin the field at a user facility, the chip identification can be read outand communicated back to the manufacturer. Then, the manufacturerperforms a search in its chip identification database 110 for thecommunicated chip identification. The chip identification can consist ofan identifier and some chip-specific information, such as hardware grade(fast or slow, for example), number of qubits in the system (17, 49, forexample), date of manufacture, manufacturing facility, etc. Whenrequesting identification, the user can be trying to ascertain if thechip 102 is authentic by verifying its existence in the manufacturer'schip identification database 110. When requesting identification, theuser can be trying to initiate communication with a third party's serverwith mediation by the manufacturer, who can authenticate the user's chipidentification as trustworthy and grant it permission to access thethird party's server upon searching for the chip identification in itschip identification database 110. When requesting identification, theuser can be deciding how to distribute workloads across the network ofexisting chips 102 and finding available quantum processors (generallyor with a particular property such as hardware grade). It should beappreciated that there are many ways to take advantage ofsuperconducting qubit chips 102 programmed with (unique) chipidentifications.

The circuit elements of the circuits 102, 104, 120, 130 can be made ofsuperconducting material. The respective resonators andtransmission/feed/probe signal lines are made of superconductingmaterials. Examples of superconducting materials (at low temperatures,such as about 10-100 millikelvin (mK), or about 4 K) include niobium,aluminum, tantalum, etc. For example, the Josephson junctions are madeof a thin tunnel barrier, such as an oxide, or a weak link, separatingtwo superconducting electrodes. The capacitors can be made ofsuperconducting material separated by a gap or a dielectric material.The transmission lines (i.e., wires) connecting the various elements aremade of a superconducting material.

Various embodiments of the present invention are described herein withreference to the related drawings. Alternative embodiments can bedevised without departing from the scope of this invention. Althoughvarious connections and positional relationships (e.g., over, below,adjacent, etc.) are set forth between elements in the followingdescription and in the drawings, persons skilled in the art willrecognize that many of the positional relationships described herein areorientation-independent when the described functionality is maintainedeven though the orientation is changed. These connections and/orpositional relationships, unless specified otherwise, can be direct orindirect, and the present invention is not intended to be limiting inthis respect. Accordingly, a coupling of entities can refer to either adirect or an indirect coupling, and a positional relationship betweenentities can be a direct or indirect positional relationship. As anexample of an indirect positional relationship, references in thepresent description to forming layer “A” over layer “B” includesituations in which one or more intermediate layers (e.g., layer “C”) isbetween layer “A” and layer “B” as long as the relevant characteristicsand functionalities of layer “A” and layer “B” are not substantiallychanged by the intermediate layer(s).

The following definitions and abbreviations are to be used for theinterpretation of the claims and the specification. As used herein, theterms “comprises,” “comprising,” “includes,” “including,” “has,”“having,” “contains” or “containing,” or any other variation thereof,are intended to cover a non-exclusive inclusion. For example, acomposition, a mixture, process, method, article, or apparatus thatcomprises a list of elements is not necessarily limited to only thoseelements but can include other elements not expressly listed or inherentto such composition, mixture, process, method, article, or apparatus.

Additionally, the term “exemplary” is used herein to mean “serving as anexample, instance or illustration.” Any embodiment or design describedherein as “exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments or designs. The terms “at least one”and “one or more” are understood to include any integer number greaterthan or equal to one, i.e. one, two, three, four, etc. The terms “aplurality” are understood to include any integer number greater than orequal to two, i.e. two, three, four, five, etc. The term “connection”can include an indirect “connection” and a direct “connection.”

References in the specification to “one embodiment,” “an embodiment,”“an example embodiment,” etc., indicate that the embodiment describedcan include a particular feature, structure, or characteristic, butevery embodiment may or may not include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

For purposes of the description hereinafter, the terms “upper,” “lower,”“right,” “left,” “vertical,” “horizontal,” “top,” “bottom,” andderivatives thereof shall relate to the described structures andmethods, as oriented in the drawing figures. The terms “overlying,”“atop,” “on top,” “positioned on” or “positioned atop” mean that a firstelement, such as a first structure, is present on a second element, suchas a second structure, wherein intervening elements such as an interfacestructure can be present between the first element and the secondelement. The term “direct contact” means that a first element, such as afirst structure, and a second element, such as a second structure, areconnected without any intermediary conducting, insulating orsemiconductor layers at the interface of the two elements.

The phrase “selective to,” such as, for example, “a first elementselective to a second element,” means that the first element can beetched and the second element can act as an etch stop.

The terms “about,” “substantially,” “approximately,” and variationsthereof, are intended to include the degree of error associated withmeasurement of the particular quantity based upon the equipmentavailable at the time of filing the application. For example, “about”can include a range of ±8% or 5%, or 2% of a given value.

As previously noted herein, for the sake of brevity, conventionaltechniques related to semiconductor device and integrated circuit (IC)fabrication may or may not be described in detail herein. By way ofbackground, however, a more general description of the semiconductordevice fabrication processes that can be utilized in implementing one ormore embodiments of the present invention will now be provided. Althoughspecific fabrication operations used in implementing one or moreembodiments of the present invention can be individually known, thedescribed combination of operations and/or resulting structures of thepresent invention are unique. Thus, the unique combination of theoperations described in connection with the fabrication of asemiconductor device according to the present invention utilize avariety of individually known physical and chemical processes performedon a semiconductor (e.g., silicon) substrate, some of which aredescribed in the immediately following paragraphs.

In general, the various processes used to form a micro-chip that will bepackaged into an IC fall into four general categories, namely, filmdeposition, removal/etching, semiconductor doping andpatterning/lithography. Deposition is any process that grows, coats, orotherwise transfers a material onto the wafer. Available technologiesinclude physical vapor deposition (PVD), chemical vapor deposition(CVD), electrochemical deposition (ECD), molecular beam epitaxy (MBE)and more recently, atomic layer deposition (ALD) among others.Removal/etching is any process that removes material from the wafer.Examples include etch processes (either wet or dry), andchemical-mechanical planarization (CMP), and the like. Semiconductordoping is the modification of electrical properties by doping, forexample, transistor sources and drains, generally by diffusion and/or byion implantation. These doping processes are followed by furnaceannealing or by rapid thermal annealing (RTA). Annealing serves toactivate the implanted dopants. Films of both conductors (e.g.,poly-silicon, aluminum, copper, etc.) and insulators (e.g., variousforms of silicon dioxide, silicon nitride, etc.) are used to connect andisolate transistors and their components. Selective doping of variousregions of the semiconductor substrate allows the conductivity of thesubstrate to be changed with the application of voltage. By creatingstructures of these various components, millions of transistors can bebuilt and wired together to form the complex circuitry of a modernmicroelectronic device. Semiconductor lithography is the formation ofthree-dimensional relief images or patterns on the semiconductorsubstrate for subsequent transfer of the pattern to the substrate. Insemiconductor lithography, the patterns are formed by a light sensitivepolymer called a photo-resist. To build the complex structures that makeup a transistor and the many wires that connect the millions oftransistors of a circuit, lithography and etch pattern transfer stepsare repeated multiple times. Each pattern being printed on the wafer isaligned to the previously formed patterns and slowly the conductors,insulators and selectively doped regions are built up to form the finaldevice. On-chip superconducting circuits described here are created byadapting the techniques of semiconductor fabrication to the formation ofneeded patterns in superconducting metal films on a semiconductorsubstrate.

The flowchart and block diagrams in the Figures illustrate possibleimplementations of fabrication and/or operation methods according tovarious embodiments of the present invention. Variousfunctions/operations of the method are represented in the flow diagramby blocks. In some alternative implementations, the functions noted inthe blocks can occur out of the order noted in the Figures. For example,two blocks shown in succession can, in fact, be executed substantiallyconcurrently, or the blocks can sometimes be executed in the reverseorder, depending upon the functionality involved.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdescribed herein.

What is claimed is:
 1. A method of forming a superconducting chipcomprising: providing resonant units having resonant frequencies,Josephson junctions being in the resonant units; and causing one or moreof the Josephson junctions to have a shorted tunnel barrier.
 2. Themethod of claim 1, wherein the shorted tunnel barrier causes an increasein the resonant frequencies for the resonant units having the shortedtunnel barrier.
 3. The method of claim 1, wherein some of the Josephsonjunctions have a tunnel barrier that is not shorted.
 4. The method ofclaim 1, wherein the resonant frequencies are designed to fall within apredefined frequency band for each of the resonant units having noshorted tunnel barrier in the Josephson junctions.
 5. The method ofclaim 4, wherein the resonant frequencies for the resonant units havingthe shorted tunnel barrier are designed to fall outside of thepredefined frequency band.
 6. The method of claim 1, wherein theJosephson junctions having the shorted tunnel barrier are predefined inadvance in order to provide a predefined chip identification.
 7. Themethod of claim 1, wherein: the Josephson junctions are structured tohave a first configuration or a second configuration; the firstconfiguration is a non-shorted Josephson junction, the non-shortedJosephson junction having a tunnel barrier defined as not being shorted;and the second configuration is a shorted Josephson junction, theshorted Josephson junction being defined as having the shorted tunnelbarrier.
 8. The method of claim 7, wherein a combination of the firstconfiguration and the second configuration defines a binaryrepresentation.
 9. The method of claim 7, wherein a combination of theresonant frequencies, associated with the first configuration and thesecond configuration, defines a binary representation.
 10. The method ofclaim 1, wherein: the resonant units having the Josephson junctionswithout the shorted tunnel barrier are configured to be read out as afirst binary number; and the resonant units having the one or more ofthe Josephson junctions with the shorted tunnel barrier are configuredto be read out as a second binary number.
 11. A method of identifying asuperconducting chip comprising: receiving first resonant frequenciesand second resonant frequencies, the first resonant frequencies being ina predefined frequency band, the second resonant frequencies beingoutside of the predefined frequency band, wherein the second resonantfrequencies each have an expected frequency location in a differentpredefined frequency band; correlating a first representation to each ofthe first resonant frequencies in the predefined frequency band; andcorrelating a second representation to the expected frequency locationfor each of the second resonant frequencies, wherein a combination ofthe first and second representations identify the superconducting chip.12. The method of claim 11, wherein: the first representation isselected from the group consisting of a number, a symbol, and a letter;and the second representation is selected from the group consisting ofanother number, another symbol, and another letter.
 13. A method ofcausing identification of a superconducting chip comprising: causing thesuperconducting chip to provide resonant frequencies from resonantunits, the resonant frequencies including first resonant frequencieswithin a predefined frequency band and second resonant frequenciesoutside of the predefined frequency band, wherein the second resonantfrequencies each have an expected frequency location in a differentpredefined frequency band; correlating a first representation to each ofthe first resonant frequencies in the predefined frequency band and asecond representation to the expected frequency location for each of thesecond resonant frequencies, wherein a combination of the first andsecond representations is a present identification of thesuperconducting chip; and determining that a previously storedidentification is a match to the present identification of thesuperconducting chip.
 14. The method of claim 13, further comprising inresponse to the match, identifying the superconducting chip having thepresent identification from different chips having differentidentifications.